A vacuum pump may be formed by positive displacement pumps such as roots or claw pumps, having one or more pumping stages connected in series. Multi-stage pumps are desirable because they involve less manufacturing cost and assembly time compared to multiple pumps in series.
Multi-stage roots or claw pumps may be manufactured and assembled in the form of a clamshell. As shown in FIG. 12, the stator 100 of such a pump comprises first and second half-shell stator components 102, 104 which together define a plurality of pumping chambers 106, 108, 110, 112, 114, 116. Each of the half-shells has first and second longitudinally extending faces which mutually engage with the respective longitudinally extending faces of the other half-shell when the half-shells are fitted together. Only the two longitudinally extending faces 118, 120 of half-shell 102 are visible in the Figure. During assembly the two half shells are brought together in a generally radial direction shown by the arrows R.
The stator 100 further comprises first and second end stator components 122, 124. When the half-shells have been fitted together, the first and second end components are fitted to respective end faces 126, 128 of the joined half-shells in a generally axial, or longitudinal, direction shown by arrows L. The inner faces 130, 132 of the end components mutually engage with respective end faces 126, 128 of the half-shells.
Each of the pumping chambers 106-116 is formed between transverse walls 134 of the half-shells. Only the transverse walls of half-shell 102 can be seen in FIG. 12. When the half-shells are assembled the transverse walls provide axial separation between one pumping chamber and an adjacent pumping chamber, or between the end pumping chambers 106, 116 and the end stator components. The present example shows a typical stator arrangement for a roots or claw pump having two longitudinally extending shafts (not shown) which are located in the apertures 136 formed in the transverse walls 134 when the half-shells are fitted together. Prior to assembly, rotors (not shown) are fitted to the shafts so that two rotors are located in each pumping chamber. Although not shown in this simplified drawing, the end components each have two apertures through which the shafts extend. The shafts are supported by bearings in the end components and driven by a motor and gear mechanism.
The multi-stage vacuum pump operates at pressures within the pumping chamber less than atmosphere and potentially as low as 10-3 mbar. Accordingly, there will be a pressure differential between atmosphere and the inside of the pump. Leakage of surrounding gas into the pump must therefore be prevented at the joints between the stator components, which are formed between the longitudinally extending surfaces 118, 120 of the half-shells and between the end faces 126, 128 of the half-shells and the inner faces 130, 132 of the end components.